1. Field of the Invention
The invention relates to a novel Edwardsiella ictaluri E-ict-VL33 strain, vaccines derived therefrom, and a method for protecting fishes using said vaccines.
2. Brief Description of the Prior Art
Edwardsiella is a type of small Gram-negative rod bacterium. The host of Edwardsiella includes Catfish, Eel, Tilapia and other warm water fishes. Edwardsiella ictaluri is one member of Edwardsiella. E. ictaluri that can infect catfish, thereby causing Edwardsiella septicemia. Acute Edwardsiella septicemia usually results in typical bacterial septicemia with a high mortality rate for fishes. Chronic Edwardsiella septicemia can result in Hole-in-the-head syndrome, septicemia or, ultimately, death.
Edwardsiella septicemia occurs worldwide and causes high mortality and considerable economic damage to the catfish industry, especially in Vietnam and the USA. Microbiological infections with E. ictaluri play a major role in catfish (Pangasianodon hypophthalmus) farming in Vietnam [Dung T T, et al., Microbe Drug Res 2008.] and were first observed in the Mekong River Delta in 1999 [Ferguson H W, et al. J Fish Dis 2001; 24:509-13]. E. ictaluri infections are seen in the USA in channel catfish (Ictalurus punctatus) and also in walking catfish in Thailand [Kasornchandra J, et al. J Fish Dis 1987; 10:137-8]. All catfish species are susceptible to E. ictaluri and the disease causes major damage in Basa Tra fish in the Mekong delta and other places where the fish is cultured in Vietnam. Many ways of protecting the fish have been tried but none so far have been very successful.
The use of antibiotics to counteract bacterial infections has been successful in some instances but not in others, especially because of widespread resistance as a result of extensive and non-controlled use of many different antibiotics either alone or in various combinations. The situation is worsened by the fact that E. ictaluri, and thus resistant E. ictaluri, can survive in the mud of a pond for up to 3 months. Furthermore, the use of antibiotics and chemotherapeutics is discouraged because of both environmental and residual problems, and long-term use of antibiotics is not a sustainable control method for fish diseases. However, preventing and protecting the fish from Edwardsiella septicemia cannot be achieved by using antibiotics which can only control the spreading of Edwardsiella septicemia.
Vaccination as a preventive measure to control infections with E. ictaluri has been tried in various catfish species. Until now, not a single vaccination or combination of vaccinations has been proven as particularly successful in the field. It has been claimed that an attenuated vaccine is able to help in the control of the disease but, to date, the product has not made major inroads in the field. Besides that, oral delivery of vaccine antigens to fish is the preferred method for several reasons. However the limitations of this method include lack of immune efficacy [Gudding R, et al. Vet Immunol Immunopathol 1999].
In view of the above-described disadvantages associated with conventional techniques, the inventor had developed a novel Edwardsiella ictaluri E-ict-VL33 strain, vaccines thereof, and a method for protecting fishes using said vaccines.